In a STN (super twisted nematic) liquid crystal display, light transmitted through a STN liquid crystal cell is colored due to the birefringence properties of a STN liquid crystal. Black and white display is therefore accomplished by erasing this coloring by means of an optical member For phase difference compensation. The above optical member that has been put to practical use in a STN liquid crystal display is a liquid crystal cell for optical compensation. However, this liquid crystal cell for optical compensation has disadvantages such as a high price, a decrease in light transmittance and the requirement of a stronger light source.
It has been therefore proposed to use a polymer film having birefringence properties and high transparency (to be referred to as "phase difference compensation film" hereinafter) as the optical member For phase difference compensation in place of the liquid crystal cell for optical compensation.
As the above phase difference compensation film, Films formed of various materials are known. As films formed of polycarbonate, there are known a film which is obtained by monoaxially stretching a transparent polycarbonate film (see JP-A-63-261302) and a film which is obtained by stretch-molding a polycarbonate of high polymerization degree having a viscosity-average molecular weight of at least 40,000 (see JP-A-3-181905).
The phase difference compensation film is generally produced by first obtaining a polymer sheet by an extrusion molding method or a solvent casting method, and stretching this film by a tension-stretching method after or while the film is heated. The birefringence difference (.DELTA.n) of the phase difference compensation film to be obtained can be adjusted by properly selecting the stretch ratio in tile length direction and the stretch ratio in the width direction at a stretching time. And, the retardation (R=.DELTA.n.t) of the phase difference compensation film to be obtained can be adjusted by adjusting fin and the film thickness (t).
However, when a general transparent polycarbonate film obtained from 4,4'-dihydroxyarylalkane (e.g., bisphenol A) is used as a phase difference compensation film as described in the above JP-A-63-261302, the retardation of the phase difference compensation film to be obtained varies to a great extent even due to a change in temperature at a stretching time. There is therefore a problem that it is difficult to stably obtain a phase difference compensation film having desired properties.
The above problem can be overcome to some extent by using a polycarbonate of high polymerization degree having a viscosity-average molecular weight of at least 40,000, obtained from bisphenol A, as is described in JP-A-3-181905. However, this polycarbonate is not yet satisfactory.
It is therefore an object of the present invention to provide a phase difference compensation film which shows a small variation in retardation caused by a temperature change at a stretching time.